Process for the separation of carbon dioxide from flue gas

ABSTRACT

A process and system for separating CO 2  from a flue gas stream is disclosed. The process involves (a) contacting a flue gas stream containing water vapor and CO 2  with an ionic absorbent under absorption conditions to absorb at least a portion of the CO 2  from the flue gas stream and form a CO 2 -absorbent complex; wherein the ionic absorbent comprises a cation and an anion comprising an amine moiety; and (b) recovering a gaseous product having a reduced CO 2  content.

PRIORITY

This application is a divisional of co-pending U.S. patent applicationSer. No. 12/930,207 filed Dec. 30, 2010, the contents of which areincorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention generally relates to a process for separating CO₂from flue gas using amine-functionalized ionic absorbents.

2. Description of the Related Art

One major source of carbon dioxide (CO₂) emission is the flue gas thatis exhausted as a result of a large industrial combustion process, e.g.,refinery heaters and boilers, steam generators, gas turbines, powerplants, etc., in large energy consuming industries such as cement, ironand steel and chemical production and oil refining. Accordingly, it isdesirable to develop a cost-effective process for separating CO₂ fromflue gases for the purpose of CO₂ capture and sequestration (CCS). Thisapproach is known as “post-combustion” CO₂ capture and is applicable toboth retrofits and new builds.

Before CO₂ can be sequestered from a large industrial source, it must becaptured in a relatively pure form. CO₂ is routinely separated andcaptured as a by-product of industrial processes such as syntheticammonia production, hydrogen (H₂) production or limestone calcination.Existing CO₂ capture technologies, however, are not cost-effective whenconsidered in the context of sequestering CO₂ from large point sources.Most large point sources use air-fired combustors, a process thatexhausts CO₂ diluted with nitrogen. For efficient carbon sequestration,the CO₂ in these exhaust gases must be separated and concentrated.

Currently, the only commercially-proven method of post-combustioncapture is through absorption of flue gas with aqueous amine solvents ina packed or trayed absorber column. CO₂ is recovered at high purity fromthe aqueous amine solvents by stream stripping in a packed or trayedregenerator column, and then cooled, dried, and compressed tosupercritical pressures for pipeline transport and eventual geologicsequestration. There are many challenges associated with using aqueousamines in “post-combustion” mode.

One such solvent is 30 wt % monoethanolamine (MEA) in water. Solventchemistry, corrosion, and viscosity considerations limit the aminestrength to about 30 wt. % MEA. At flue-gas CO₂ partial pressures (e.g.,0.04 to 0.15 atm), the CO₂-rich (“rich”) solvent loading is about 0.42to 0.45 mol CO₂/mol MEA and the CO₂-lean (“lean”) solvent loading isabout 0.15 to 0.17 mol CO₂/mol MEA. The difference in loading (0.25 to0.3 mol CO₂/mol MEA) sets the circulation rate of the amine andinfluences capital and operating costs.

MEA also has disadvantages in that it has several mechanisms of loss,and a continuous makeup of MEA is required by post-combustion processes.For example, MEA degrades in the presence of oxygen from the flue gas.Thus, to limit the oxidative degradation, corrosion inhibitors may beused. MEA also degrades into heat-stable salts (HSS) from reaction withCO₂. To solve this problem, a reclaimer would be added on theregenerator to separate the HSS from the amine solution to providesuitable makeup MEA. Lastly, the volatility of MEA results in thetreated flue gas to contain in excess of 500 ppmv MEA when leaving theabsorber to the vent. To address this, a wash section is added at thetop of the absorber and makeup water is added to scrub the MEA from thetreated flue gas. The mixture is then sent down the column along withthe remaining lean solvent to absorb CO₂ from the incoming flue gas.Water washing can cut the MEA emissions to about 3 ppmv.

MEA may also degrade over time thermally, thereby limiting thetemperature of operation in the absorber and regenerator. With a cooledflue gas inlet temperature of about 56° C., the absorber column mayoperate at a bottoms temperature of 54° C. and a pressure of 1.1 barwhile the regenerator may operate at a bottoms temperature of 121° C. (2bar saturated steam) at 1.9 bar. For 30 wt. % MEA, the amine reboilersteam temperature is kept at less than 150° C. (4.7 bar saturated steam)to limit thermal degradation.

MEA also degrades in the presence of high levels of NOx and SOx whichare common in facilities that burn coal and fuel oil. However, if CO₂removal from a high NOx and SOx containing flue gas is desired, separateprocess facilities such as SCR (Selective Catalytic Reduction) and FGD(Flue Gas Desulfurization) are needed for removal of NOx and SOx,respectively. In order to have a superior, post-combustion CO₂ removaltechnology that is better than those known in the art (30 wt. % MEA andsimilar aqueous amines), an improved CO₂-removal solvent and processusing the solvent is required.

Ionic Liquids (IL) are a class of compounds that are made up entirely ofions and are liquid at or below process temperatures. ILs are known inthe art to have vanishingly low or negligible vapor pressure, and areoften studied as candidates for environmentally-benign solvents,catalysts, and gas and liquid-phase separation agents. Many ILs known inthe art behave as physical solvents, meaning that the loading capacityof CO₂ is linear with the equilibrium partial pressure of CO₂. These ILsare unsuitable for flue-gas applications since the partial pressure ofCO₂ is from about 0.04 to about 0.15 atm and are better suited forhighly-concentrated CO₂ applications such as syngas. Other ILs appearmore active for CO₂ at lower partial pressures and have non-linearloading curves, for example, bmim acetate as disclosed in U.S. Pat. No.7,527,775. The anions of this type of ILs are able to interact favorablywith CO₂ and thereby have higher loadings at low partial pressure.

More recently, researchers have developed task-specific IL (TSIL) andother forms of functionalized ILs that have chemical functional groupdesigned to interact even more strongly and/or specifically with CO₂.Although these amine-functionalized ILs are known to have a highcapacity for CO₂, they are also known to be far more viscous compared tonon-functionalized ILs. However, a high viscosity is undesirable on acommercial scale, because it means that pumping costs are higher, masstransfer kinetics between gas and liquid are lower (resulting in tallercolumns and more packing material), and the efficiency of heat transferis reduced (needed for eventual regeneration).

Accordingly, there is a continued need for improved systems andprocesses for removing CO₂ from flue gases that can be carried out in asimple, cost-effective manner.

SUMMARY

In accordance with one embodiment of the present invention, a processfor separating carbon dioxide (CO₂) from a flue gas stream, the processcomprising (a) contacting a flue gas stream containing water vapor andCO₂ with an ionic absorbent under absorption conditions to absorb atleast a portion of the CO₂ from the flue gas stream and form aCO₂-absorbent complex; wherein the ionic absorbent comprises a cationand an anion comprising an amine moiety; and (b) recovering a gaseousproduct having a reduced CO₂ content.

In accordance with a second embodiment of the present invention, asystem is provided which comprises:

(a) a supply of a flue gas stream containing water vapor and CO₂;

(b) a supply of an ionic absorbent comprising a cation and an anioncomprising an amine moiety; and

(c) an absorption unit for contacting the flue gas stream and the ionicabsorbent under absorption conditions to absorb at least a portion ofthe CO₂ from the flue gas stream and form a CO₂-absorbent complex and agaseous product having a reduced CO₂ content.

The process of the present invention employs amine-functionalized ionicabsorbent for post-combustion CO₂ capture from a flue gas stream. Theprocess can be carried out without cooling the flue gas stream and atrelatively low pressures. This is a significant advantage which allowsfor the process to be carried out in a simple, cost effective manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 shows generic synthetic schemes for preparing an ionicabsorbent in accordance with the present invention.

FIG. 5 shows a process flow diagram scheme for the removal of CO₂ from aflue gas stream in accordance with one embodiment of the presentinvention.

FIG. 6 shows the CO₂ loading results at various temperature foramine-functionalized ionic absorbents in accordance with the presentinvention.

FIG. 7 shows the CO₂ loading results at various temperature formonoethanolamine in 70 wt. % water.

DETAILED DESCRIPTION

The present invention is directed to a process for separating CO₂ from aflue gas stream. In one embodiment, the process involves (a) contactinga flue gas stream containing water vapor and CO₂ with an ionic absorbentunder absorption conditions to absorb at least a portion of the CO₂ fromthe flue gas stream and form a CO₂-absorbent complex; wherein the ionicabsorbent comprises a cation and an anion comprising an amine moiety;and (b) recovering a gaseous product having a reduced CO₂ content.

The Flue Gas Stream

The flue gas stream for use in the process of the present invention maybe any flue gas stream that is generated from a combustion apparatussuch as a refinery plant, industrial power plant, etc. In oneembodiment, the flue gas stream is from a stack that removes flue gasdischarged from an industrial facility to the outside. Representativeexamples of such flue gas streams includes a gas turbine flue gas, afurnace flue gas, a hot oil furnace flue gas, a steam generator fluegas, a preheater flue gas, a reformer flue gas, steam methane reformerflue gas, FCC (fluid catalytic cracker) regenerator flue gas, CFB(circulating fluid bed boiler) and the like. The flue gas streamcontains at least some amount of water vapor and CO₂. In one embodiment,the flue gas stream contains fully saturated water. In anotherembodiment, the flue gas stream contains about 50% up to 100% humidity.In general, the amount of CO₂ present in the flue gas will depend fromits source. For example, for a flue gas from a combined cycle gasturbine, the flue gas stream contains about 3.5 mol % CO₂.Alternatively, for a flue gas from steel production or cement kilns, theflue gas stream contains about 30 mol % CO₂. Accordingly, in oneembodiment, the flue gas stream contains from about 3.5 mol % to about30 mol % CO₂. In another embodiment, the flue gas stream contains fromabout 4 mol % to about 15 mol % CO₂. In another embodiment, the flue gasstream contains from about 10 mol % to about 15 mol % CO₂.

In general, the flue gas stream is typically a hot flue gas stream,i.e., a flue gas stream having a temperature of at least about 80° C.(175° F.). In another embodiment, the flue gas stream has a temperatureranging from about 80° C. to about 150° C. (300° F.). While thetemperature of the flue gas stream can always be higher, for practicalreasons this is generally not the case unless the original combustiondevice (e.g., heater, boiler, etc.) was poorly designed and highlyin-efficient. In addition, the flue gas stream can have a lowertemperature in the case where a configuration that includes pre-coolingof the flue gas or input from a cooled gas sources such as wet FGDeffluent is used.

The Ionic Absorbent

The ionic absorbent is generally composed of a cation and an anion. Inone embodiment, the ionic absorbent is a liquid ionic absorbent andincludes a category of compounds which are made up entirety of ions andare liquid at or below process temperatures including room temperature.The ionic liquids may have low melting points, for example, from −100°C. to 200° C. They tend to be liquid over a very wide temperature range,with a liquid range of up to about 500° C. or higher, ionic liquids aregenerally non-volatile, with effectively no vapor pressure. Many are airand water stable, and can be good solvents for a wide variety ofinorganic, organic, and polymeric materials. In another embodiment, theionic absorbent is a solid ionic absorbent and includes a category ofcompounds which are made up entirely of ions and are solid in ananhydrous state at room temperature.

The properties of the ionic absorbent can be tailored by varying thecation and anion pairing. The ionic absorbent for use in the system andprocess of the present invention includes a cation and an anioncomprising an amine moiety. The amine moiety advantageously providesselectivity for the ionic absorbent to complex with CO₂, especially atthe low partial pressure of CO₂ typical in flue gas. The presence ofthis amine moiety on the anion rather than the cation of the ionicabsorbent facilitates reaction of the ionic liquid with CO₂ at ratios ofone mol of CO₂ per mol of ionic absorbent.

It is believed that the cation group of the ionic absorbent has littleimpact on the molar CO₂ absorption capacity. Accordingly, the molecularweight of the cation can be as low as possible thereby lowering theoverall molecular weight of the ionic absorbent. In one embodiment, themolecular weight of the cation can range from about 18 to about 500atomic mass unit (AMU) (g/mole). In another embodiment, the molecularweight of the cation can range from about 18 to about 400 atomic massunit (AMU) (g/mole).

In one embodiment, a cation is a secondary, tertiary or quaternaryphosphonium cation represented by the general formula:

wherein R is the same or different and is hydrogen, a substituted orunsubstituted alkyl group, a substituted or unsubstituted fluoroalkylgroup, a substituted or unsubstituted cycloalkyl group, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, a substituted or unsubstituted arylalkyl group, a substituted orunsubstituted heteroarylalkyl group, or —(CH₂)_(n)—R′, wherein R′represents independently for each occurrence a substituted orunsubstituted cycloalkyl group, a substituted or unsubstituted arylgroup, or a substituted or unsubstituted heteroaryl group; and nrepresents independently for each occurrence an integer in the range 1to 10 inclusive.

In one embodiment, a cation is a secondary, tertiary or quaternaryammonium cation represented by the general formula:

wherein R is the same or different and is hydrogen, a substituted orunsubstituted alkyl group, a substituted or unsubstituted fluoroalkylgroup, a substituted or unsubstituted cycloalkyl group, a substituted orunsubstituted aryl group, a substituted or unsubstituted heteroarylgroup, a substituted or unsubstituted arylalkyl group, a substituted orunsubstituted heteroarylalkyl group, or —(CH₂)_(n)—R′, wherein R′represents independently for each occurrence a substituted orunsubstituted cycloalkyl group, a substituted or unsubstituted arylgroup, or a substituted or unsubstituted heteroaryl group, or three Rgroups together with the nitrogen, atom to which they are bonded can betaken together to represent pyridinium, imidazolium, benzimidazolium,pyrazolium, benzpyrazolium, indazolium, thiazolium, benzithiazolium,oxazolium, benzoxazolium, isoxazolium, isothiazolium, imdazolidenium,guanidinium, quinuclidinium, triazolium, tetrazolium, quinolinium,isoquinolinium, piperidinium, pyrrolidinium, morpholinium, pyridazinium,pyrazinium, piperazinium, triazinium, azepinium, or diazepinium; and nrepresents independently for each occurrence an integer in the range 1to 10 inclusive.

In one embodiment, the cation is a Group 1 or Group 2 metal of thePeriodic Table. Representative examples of Group 1 metals includelithium, sodium, potassium, rubidium, cesium and the like.Representative examples of Group 2 metals include calcium, barium,magnesium, or strontium and the like.

In one embodiment, a cation includes, but is not limited to, a Group 1or Group 2 metal of the Periodic Table, an ammonium cation, phosphoniumcation, an imidazolium cation, a pyridinium cation, a pyrazolium cation,an oxazolium cation, a pyrrolidinium cation, a piperidinium cation, analkyl thiazolium cation, an alkyl guanidinium cation, a morpholiniumcation, a trialkylsulfonium cation, a triazolium cation, and the like.

In one embodiment, a cation is a trialkyl or a tetraalkyl ammoniumcation or phosphonium cation in which the alkyl group of the trialkyl ortetraalkyl is the same or different and is a C₁ to C₃₀ straight orbranched, substituted or unsubstituted alkyl group. In anotherembodiment, a cation is a tetraalkyl ammonium cation or a tetraalkylphosphonium cation in which the alkyl group of the tetraalkyl is thesame or different and is a C₁ to C₆ straight or branched, substituted orunsubstituted alkyl group. The cation may contain ring structures wherethe N or P atom is a part of the ring structure.

Suitable anions for the ionic absorbent include those represented by thegeneral formula:

R¹—N(R¹)-(L)-A⁻

wherein R¹ is the same or different and includes hydrogen, a straight orbranched C₁ to C₃₀ substituted, or unsubstituted alkyl group, a C₁ toC₂₀ ester-containing group, a substituted, or unsubstituted C₃ to C₃₀cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group,a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substitutedor unsubstituted C₅ to C₃₀ heteroaryl group, a substituted orunsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₅ to C₃₀ heteroarylalkyl group, or R and R¹ together withthe nitrogen atom to which they are bonded are joined together to form aheterocyclic group; L is a linking group, which can be a bond, or adivalent group selected from the group consisting of a straight orbranched C₁ to C₃₀ substituted or unsubstituted alkyl group, a C₁ to C₂₀ester-containing group, a substituted or unsubstituted C₃ to C₃₀cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group,a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substitutedor unsubstituted C₅ to C₃₀ heteroaryl group, a substituted orunsubstituted C₃ to C₃₀ heterocyclic ring, a substituted orunsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted orunsubstituted C₆ to C₃₀ heteroaryl alkyl group and the like; and A⁻ isan anionic moiety.

In one embodiment, A⁻ is SO₃ ⁻ or PO₄ ⁻ or a conjugate base ofmultivalent acid.

In one embodiment, R¹ are the same or different and include hydrogen, ora straight or branched C₁ to C₆ substituted or unsubstituted alkylgroup, L is a divalent straight or branched C₁ to C₆ substituted orunsubstituted alkyl group and A⁻ is SO₃ ⁻.

Representative examples of alkyl groups for use herein include, by wayof example, a straight or branched alkyl chain containing carbon andhydrogen atoms of from 1 to about 30 carbon atoms and preferably from 1to about 6 carbon atoms with or without unsaturation, to the rest of themolecule, e.g., methyl, ethyl, n-propyl, 1-methylethyl (isopropyl),n-butyl, n-pentyl, etc. and the like.

Representative examples of fluoroalkyl groups for use herein include, byway of example, a straight or branched alkyl group as defined hereinhaving one or more fluorine atoms attached to the carbon atom, e.g.,—CF₃, —CF₂CF₃, —CH₂C₃, —CH₂CF₂H, —CF₂H and the like.

Representative examples of substituted or unsubstituted cycloalkylgroups for use herein include, by way of example, a substituted orunsubstituted non-aromatic mono or multicyclic ring system of about 3 toabout 20 carbon atoms such as, for example, cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, bridged cyclic groups or sprirobicyclic groups,e.g., spiro-(4,4)-non-2-yl and the like, optionally containing one ormore heteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted cycloalkylalkylgroups for use herein include, by way of example, a substituted orunsubstituted cyclic ring-containing group containing from about 3 toabout 20 carbon atoms directly attached to the alkyl group which arethen attached to the main structure of the monomer at any carbon fromthe alkyl group that results in the creation of a stable structure suchas, for example, cyclopropylmethyl cyclobutylethyl cyclopentylethyl andthe like, wherein the cyclic ring can optionally contain one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted cycloalkenylgroups for use herein include, by way of example, a substituted orunsubstituted cyclic ring-containing group containing from about 3 toabout 20 carbon atoms with at least one carbon-carbon double bond suchas, for example, cyclopropenyl, cyclobutenyl, cyclopentenyl and thelike, wherein the cyclic ring can optionally contain one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted aryl groups foruse herein include, by way of example, a substituted or unsubstitutedmonoaromatic or polyaromatic group containing from about 5 to about 20carbon atoms such as, for example, phenyl naphthyl, tetrahydronapthyl,indenyl, biphenyl and the like, optionally containing one or moreheteroatoms, e.g., O and N, and the like.

Representative examples of substituted or unsubstituted arylalkyl groupsfor use herein include, by way of example, a substituted orunsubstituted aryl group as defined herein directly bonded to an alkylgroup as defined herein, e.g., —CH₂C₆H₅, —C₂H₅C₆H₅ and the like, whereinthe aryl group can optionally contain one or more heteroatoms, e.g., Oand N, and the like.

Representative examples of fluoroaryl groups for use herein include, byway of example, an aryl group as defined herein having one or morefluorine atoms attached to the aryl group.

Representative examples of ester groups for use herein include, by wayof example, a carboxylic acid ester having one to 20 carbon atoms andthe like.

Representative examples of heterocyclic ring groups for use hereininclude, by way of example, a substituted or unsubstituted stable 3 toabout 30 membered ring group, containing carbon atoms and from one tofive heteroatoms, e.g., nitrogen, phosphorus, oxygen, sulfur andmixtures thereof. Suitable heterocyclic ring groups for use herein maybe a monocyclic, bicyclic or tricyclic ring system, which may includefused, bridged or spiro ring systems, and the nitrogen, phosphorus,carbon, oxygen or sulfur atoms in the heterocyclic ring group may beoptionally oxidized to various oxidation states. In addition, thenitrogen atom may be optionally quaternized; and the ring radical may bepartially or fully saturated (i.e., heteroaromatic or heteroarylaromatic). Examples of such heterocyclic ring functional groups include,but are not limited to, azetidinyl, acridinyl, benzodioxolyl,benzodioxanyl, benzofurnyl, carbazolyl, cinnolinyl, dioxolanyl,indolizinyl, naphthyridinyl, perhydroazepinyl, phenazinyl,phenothiazinyl, phenoxazinyl, phthalazinyl, pyridyl, pteridinyl,purinyl, quinazolinyl, quinoxalinyl, quinolinyl, isoquinolinyl,tetrazoyl, imidazolyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl,piperidinyl, piperazinyl, 2-oxopiperazinyl, 2-oxopiperidinyl,2-oxopyrrolidinyl, 2-oxoazepinyl, azepinyl, pyrrolyl, 4-piperidonyl,pyrrolidinyl, pyrazinyl, pyrimidinyl, pyridazinyl, oxazolyl, oxazolinyl,oxazolidinyl, triazolyl, indanyl, isoxazolyl, iso-oxazolidinyl,morpholinyl, thiazolyl, thiazolinyl, thiazolidinyl, isothiazolyl,quinuclidinyl, isothiazolidinyl, indolyl, isoindolyl, indolinyl,isoindolinyl, octahydroindotyl, octahydroisoindolyl, quinolyl,isoquinolyl, decahydroisoquinolyl benzimidazolyl, thiadiazolyl,benzopyranyl, benzothiazolyl, benzooxazolyl, furyl, tetrahydrofurtyl,tetrahydropyranyl, thienyl, benzothienyl, thiamorpholinyl,thiamorpholinyl sulfoxide, thiamorpholinyl sulfone, dioxaphospholanyl,oxadiazolyl, chromanyl, isochromanyl and the like and mixtures thereof.

Representative examples of heterocycloalkyl groups for use hereininclude, by way of example, a substituted or unsubstituted heterocyclicring group as defined herein directly bonded to an alkyl group asdefined herein. The heterocycloalkyl moiety may be attached to the mainstructure at carbon atom in the alkyl group that results in the creationof a stable structure.

Representative examples of heteroaryl groups for use herein include, byway of example, a substituted or unsubstituted heterocyclic ring groupas defined herein. The heteroaryl ring radical may be attached to themain structure at any heteroatom or carbon atom that results in thecreation of a stable structure.

Representative examples of heteroarylalkyl groups for use hereininclude, by way of example, a substituted or unsubstituted heteroarylring group as defined herein directly bonded to an alkyl group asdefined herein. The heteroarylalkyl moiety may be attached to the mainstructure at any carbon atom from the alkyl group that results in thecreation of a stable structure.

It will be understood that the term “substituted with” includes theimplicit proviso that such substitution is in accordance with permittedvalence of the substituted atom and the substituent, and that thesubstitution results in a stable compound, e.g., which does notspontaneously undergo transformation such as by rearrangement,cyclization, elimination, or other reaction. Representative examples ofsuch substituents include, but are not limited to, hydrogen, fluorine,hydroxyl groups, halogen group, carboxyl groups, cyano groups, nitrogroups, oxo (═O), thio(═S), substituted or unsubstituted alkyl,substituted or unsubstituted fluoroalkyl substituted or unsubstitutedalkoxy, substituted or unsubstituted alkenyl, substituted orunsubstituted alkynyl, substituted or unsubstituted aryl, substituted orunsubstituted arylalkyl substituted or unsubstituted cycloalkylsubstituted or unsubstituted cycloalkenyl, substituted or unsubstitutedamino, substituted or unsubstituted aryl, substituted or unsubstitutedheteroaryl, substituted heterocycloalkyl ring, substituted orunsubstituted heteroarylalkyl, substituted or unsubstituted heterocyclicring, and the like.

Representative examples of ionic absorbents for use in the aqueous ionicabsorbent solutions of the present invention includes tetrabutylammoniumN-propyl-N-(3-sulfopropyl)amine, tetrabutylphosphoniumN-isopropyl-N-(3-sulfopropyl)amine, tetraethylammoniumN-isopropyl-N-(3-sulfopropyl)amine and the like and mixtures thereof.

The ionic absorbents for use in the process of the present invention areknown and can be prepared by methods known in the art, e.g., asdisclosed in U.S. Pat. Nos. 7,208,605 and 7,744,838 and WO 2008/122030,the contents of which are incorporated by reference herein. For example,in one embodiment, the ionic absorbents can be prepared in accordancewith the synthetic schemes generally represented in FIGS. 1 and 2(reactions with primary amines, secondary amines or diamines) via azwitterionic intermediate. In one embodiment, the ionic absorbents canbe prepared in accordance with the synthetic scheme generallyrepresented in FIG. 3 in which the zwitterionic intermediate can bereacted with an epoxide. In one embodiment, the ionic absorbents can beprepared in accordance with, the synthetic scheme generally representedin FIG. 4.

While the above examples have shown the reaction of the zwitterionicintermediates with ammonium hydroxide salts, as the base, one may alsouse other cations as described above, e.g., a phosphonium cation, aheterocyclic (e.g., imidazolium or pyridinium) cation, alkali metalcation or an alkaline earth metal cation as the counterion. In certainembodiments, the cations can be metal cations, such as Na, K, Ca, Ba,etc.

While the selected synthetic routes described above have all suggestedreacting hydroxide salts of various cations with the zwitterions, othersynthetic approaches can be envisioned as well, such as zwitteriondeprotonation with strong bases like NaH or BuLi, followed by an ionmetathesis step to exchange the Na or Li for a different cation.

As one skilled in the art will readily appreciate, in the case where theflue gas contains sufficient water vapor, the ionic absorbent can beused as is, i.e., neat. In other words, when there is a sufficientamount of water vapor in the flue gas, the sufficient amount of waterand ionic absorbent will form an aqueous ionic absorbent solution havinga viscosity suitable for use in the process of the present invention. Asuitable viscosity for the aqueous ionic absorbent solution containingthe ionic absorbent in the process of the present invention willordinarily be from about 0.1 to about 100 centistoke (cSt). in oneembodiment, a suitable viscosity for the aqueous ionic absorbentsolution containing the ionic absorbent in the process of the presentinvention can range from about 0.5 to about 40 cSt.

Alternatively, in the case where the flue gas does not containsufficient water vapor, a sufficient amount of a diluent can be added tothe aqueous ionic absorbent solution containing the ionic absorbent toreduce its viscosity to the suitable viscosity discussed above. Asuitable diluent includes, by way of example, inert diluents such aswater, monohydric alcohols, polyols, and the like and mixtures thereof.Representative examples of suitable monohydric alcohols include C₁ toC₁₂ alcohols such as methanol, ethanol, isopropanol, 1-propanol,1-butanol, 2-butanol, t-butanol, 2-methyl-1-propanol, 1-pentanol,1-hexanol, 1-heptanol, 4-heptanol, 1-octanol, 1-nonyl alcohol,1-decanol, 1-dodecanol and the like and mixtures thereof.

The polyols for use as a diluent include those having from 2 to about 10carbon atoms and from two to six hydroxyl groups. Representativeexamples of suitable polyols include glycerol, triethylene glycol,2-ethylene glycol, diethylene glycol, 1,2-propylene glycol,1,3-propylene glycol, tetraethylene glycol, hexylene glycol and the likeand mixtures thereof. In one preferred embodiment, the diluent is water.

In order for the aqueous solutions containing the ionic absorbent tohave a larger volumetric absorption capacity for CO₂ (mol CO₂/cm³solvent) over an aqueous solution of 30 wt. % water and MEA, themolecular weight of the neat ionic absorbent should be as low aspossible, e.g., a molecular weight of no more than about 700 atomic massunit (AMU) (g/mole). In one embodiment, the molecular weight of the neationic absorbent is from about 75 to about 700 AMU (g/mole). In oneembodiment, the molecular weight of the neat ionic absorbent is belowabout 600 AMU (g/mole). In another embodiment, the molecular weight ofthe neat ionic absorbent is from about 75 to about 600 AMU (g/mole). Inone embodiment, the molecular weight of the neat ionic absorbent isbelow about 500 AMU (g/mole). In another embodiment, the molecularweight of the neat ionic absorbent is from about 75 to about 500 AMU(g/mole).

The amount of water and optional diluent to achieve sufficient viscosityand to provide enough stripping stream in regenerator (H), as discussedbelow, should be no more than about 80 wt. %, based on the total weightof the aqueous solution. In another embodiment, the amount of water andoptional diluent can range from about 15 to about 80 wt. %, based on thetotal weight of the aqueous solution.

It is believed that the ionic absorbents are stable under the O₂, NO_(x)and SO_(x) which may be present in the flue gas. Therefore, the use ofthe ionic absorbent according to the present invention may avoid theneed for SCR (Selective Catalytic Reduction) and/or FGD (Flue GasDesulfurization) especially in refinery sources. In addition, theoverall process may operate in a multi-pollutant mode where the aqueoussolutions containing the ionic absorbent co-absorb one or more of SO₂,COS, NO_(x), COS, and SO_(x) in the flue gas. Thus, the aqueoussolutions containing the ionic absorbent for use herein may be employedto capture all or some of the pollutants in addition to CO₂ which arepresent in the flue gas.

Process

FIG. 5 illustrates a process scheme according to one embodiment of thepresent invention. In general, FIG. 5 includes a carbon dioxide (CO₂)separation system for recovering high purity CO₂ from flue gas stream(A). First, flue gas stream (A) enters blower (B), which blows flue gasstream (A) to absorber (D). Blower (B) can be any type of blower knownto one skilled in the art. Generally, the pressure of flue gas stream(A) is around 1 bar. Blower (B) raises the pressure of the flue gasstream to a pressure ranging from L1 to about 1.5 bar. The pressure istypically raised to overcome the pressure drop associated with flowingthe gas through the absorber tower.

In one embodiment, absorber (D) is a packed tower. The packing may berandom packing, structured packing or any combination thereof. Therandomly packed material can include, but is not limited to, Raschigrings, saddle rings, Pall rings, or any other known type of packingring, or combination of packing rings. The structured packed materialcan include, but is not limited to, corrugated sheets, crimped sheets,gauzes, grids, wire mesh, monolith honeycomb structures, or anycombination thereof. Examples of structured packing include Sulzer DX™,Mellapak™, Mellapak Plus™, Katapak™, and the like.

As discussed above, water present in the flue gas stream will form anaqueous solution containing the ionic absorbent. If the viscosity of theaqueous solution is not sufficient for use in the process of the presentinvention, then it may be necessary to add one or more diluents to theaqueous solution to further reduce its viscosity to a viscosity suitablefor the process of the present invention. Accordingly, a diluent stream(G) is present at the top of the column to further hydrate the incomingionic liquid stream (C) and reduce its viscosity as discussed above. Inaddition, diluent stream (G) can scrub any additional diluent that maybecarried over into treated flue gas stream (A1) leaving the absorber (D)to partly or completely remove any unwanted impurities. Typically, thetotal amount of water and diluents, when present, in the ionic absorbentis largely dictated by viscosity requirements, and is generally lessthan about 80 wt. %. In one embodiment, the total amount of waterpresent is from about 15 wt. % to about 80 wt. %.

The flue gas stream (A) and ionic absorbent stream (C) are contacted inabsorber (D). In general, fine gas stream (A) is introduced intochemical absorber (D) and during the process of flow from the bottom up,the carbon dioxide in flue gas stream (A) is absorbed by ionic absorbent(C) flowing from the top down. The end gas, i.e., treated flue gasstream (A1), which is essentially depleted of carbon dioxide, isintroduced out of the absorber, for example, through a vent from the topof the absorber after contacting (or being scrubbed) by diluent streamG. In one embodiment, from about 80 to about 95% of CO₂ in stream A hasbeen removed to form stream A1.

The ionic absorbent stream (E), which has absorbed carbon dioxide(CO₂-rich aqueous ionic absorbent solution stream), comes out ofabsorber (D) from the bottom and is pumped to cross exchanger (K), whereits temperature is raised; and then the preheated stream (F) may bepumped through pump (W2). Pump (W2) raises the pressure of ionicabsorbent stream (E) to greater than about 1.9 bar to providepressurized ionic absorbent stream (F1). In one embodiment, the pressureof ionic absorbent stream (E) is raised to a pressure of greater thanabout 1.9 bar to about 10 bar. In one embodiment, the pressure is raisedto about 4 bar to about 8 bar. Pump (W2) can be any apparatus capable ofraising the pressure of preheated stream (F) such as, for example, acentrifugal pump, a reciprocating pump, a gear pump, etc.

Next, pressurized ionic absorbent stream (F1) is sent to the top ofregenerator (H) (i.e., stripper) to flow down through the regeneratorwhere it is former heated to a temperature ranging from, about 90° C. toabout 200° C. by, for example, steam-heated or hot-oil heated reboilerand under vacuum, atmospheric or high pressure conditions. In oneembodiment, pressurized ionic absorbent stream (F1) is sent to the topof regenerator (H) to flow down through the regenerator where it isfurther heated to a temperature of about 120° C. to about 180° C. Inthis manner, most of the carbon dioxide in the ionic liquid (F1) isreleased as a wet carbon dioxide gas stream (L) and emitted out from thetop of regenerator (H). Regenerator (H) is a packed tower and can be anyrandom or structure packing as discussed above with absorber (D). Thehigh temperature regeneration allows for greater flexibility in usingdifferent levels of steam and/or waste heat resources in reboiler (Q),as discussed below. The regenerator (H) is typically run at relativelyhigh pressure, e.g., a pressure from about 1.9 bar to about 10 bar. Byrunning regenerator (H) at a relatively high pressure, carbon dioxidestream (L) may be recovered at higher pressure thereby reducing thecapital and operating costs associated with carbon dioxide compression(Y), discussed below. The ionic absorbent depleted of carbon dioxide,i.e., ionic absorbent stream (S), is emitted out from the bottom ofregenerator (H).

In one embodiment, when flue gas stream (A) is first pre-cooled to atemperature of about 40° C. to about 60° C. and then sent to theabsorber and contacted with ionic absorbent stream (C), the desorptionconditions for removing carbon dioxide from ionic absorbent stream (F1)may include heating stream (F1) to a temperature of greater than 60° C.to about 80° C. in the case where the regenerator (i.e., stripper) isrun at vacuum conditions. In another embodiment, when flue gas stream(A) is first pre-cooled to a temperature of about 40° C. to about 60° C.and then sent to the absorber and contacted with ionic absorbent stream(C), the desorption conditions for removing carbon dioxide frompressurized ionic absorbent stream (F1) may include heating stream (F1)to a temperature of greater than 100° C. to about 120° C. in the casewhere the regenerator is run at atmospheric conditions. In yet anotherembodiment, when flue gas stream (A) is first pre-cooled to atemperature of about 40° C. to about 60° C. and then sent to theabsorber and contacted with ionic absorbent stream (C), the desorptionconditions for removing carbon dioxide from pressurized ionic absorbentstream (F1) may include heating stream (F1) to a temperature of about120° C. to about 200° C. in the case where the regenerator is run athigh-pressure conditions (i.e., 1.9 to 10 bar).

In one embodiment, when flue gas stream (A) is sent to the absorber at atemperature of about 60° C. to about 80° C., the desorption conditionsfor removing carbon dioxide from pressurized ionic absorbent stream (F1)may include heating stream (F1) to a temperature of about 100° C. toabout 200° C. in the case where the regenerator is run at atmospheric orhigh-pressure conditions.

In one embodiment, when flue gas stream (A) is sent to the absorber at atemperature of about 80° C. to about 100° C. the desorption conditionsfor removing carbon dioxide from pressurized ionic absorbent stream (F1)may include heating stream (F1) to a temperature of about 120° C. toabout 200° C. in the case where the regenerator is run at atmospheric orhigh-pressure conditions.

The wet carbon dioxide gas stream (L) coming from the top of regenerator(H) is then passed through cooler (M), such as a condenser, where it iscooled to provide cooled carbon dioxide gas stream (N). The temperatureof carbon dioxide gas stream (L) is generally decreased to about 30° C.to about 50° C. The cooled carbon dioxide gas stream (N) is sent toseparator (O) where condensed water and trace amounts of ionic absorbentand optional diluents is separated and returned to regenerator (H) asreflux (P1). The gas stream (R), as the carbon dioxide product, is sentto gas injection regulatory system (X) where it is dehydrated usingmethods known in the art, e.g., triethylene glycol dehydration orheatless absorption using molecular sieves. The dehydrated gas is thensent to compressor (Y) to be pressurized, under normal pressure and atemperature of less than about 60° C., to about 7.4 MPa or higher, andsent off to pipeline (Z). In one embodiment, the captured CO₂ can beused on-site or can be made available for sale to a co-located facility.Dried CO₂ will be compressed in a series of compressors and intercoolersto a final temperature of about 40° C. to about 60° C. The lastcompression stage would be close to the supercritical pressure of CO₂(i.e., about 1100 psig). Once CO₂ is supercritical, it may be pumped asa dense phase fluid to any pressure required for transportation—finaldense phase pressure may range from about 100 to about 200 bar.

The reboiler (Q) is a shell and tube heat exchanger, ionic absorbentstream (S) coming from the bottom of the regenerator (H) enters into thetubes of the reboiler (Q) where it is heated by steam in the shell-sideof the reboiler. Stream (T) is the supply heating medium, such as steamthat it is available from the facility generating the fine gas (e.g.,refinery, gas/oil-fired boiler, power plant, etc) while stream (U) isthe return condensate that is returned back to the utility system of thefacility. Therefore, the ionic absorbent stream (S) is heated in thereboiler (Q) and at least a portion of the carbon dioxide and watervapor present therein is released out and leaves from the top ofreboiler (Q) into regenerator (H) as stripping gas (I). On the otherhand, ionic absorbent solution (J) with significantly decreased contentof absorbed carbon dioxide (also referred to as “CO₂-lean absorbentsolution”) is sent back to cross exchanger (K).

The following non-limiting examples are illustrative of the presentinvention.

Experiments were conducted to measure the CO₂ absorption of ionicabsorbent materials diluted with water and to demonstrate theireffectiveness.

The equilibrium CO₂ carrying capacity of the aqueous ionic absorbentsolutions was measured via a volumetric method. A known quantity ofionic absorbent-water mixture is injected into a sealed pressure vesselcontaining high purity CO₂ gas at pressure of approximately 15 psia, andthe vessel is shaken to provide mixing of the solution. The pressure inthe vessel decreases as CO₂ is absorbed into the solution, and thepressure is monitored until the system reaches an equilibrium pressure.The temperature of the system is controlled and monitored, and anequilibrium pressure is measured for multiple temperatures in eachexperiment. The initial and final pressure of CO₂, the volume of thepressure vessel, and the quantity and composition of the injected ionicabsorbent-water mixture are all known.

The measured experimental data are used to calculate the loading of CO₂in the solution, which is reported as moles of CO₂ bound per mol ofionic absorbent in the solution. An aqueous ionic absorbent solution iscontacted in a sealed vessel with a low partial pressure CO₂ gassimulating a flue gas stream. The vessel is maintained at a constanttemperature by way of heating tape and is shaken to allow good contactbetween CO₂ and the absorbent solution until the system reachesequilibrium. When the pressure reaches the steady state, then a pressureis recorded and the vessel temperature is changed to the next set point.Based on the equilibrium CO₂ pressure, the loading per mole of ionicabsorbent is calculated. The measured pressure in the vessel is used todetermine the moles of gaseous CO₂ present in the vessel before andafter CO₂ absorption occurs (before and after the solvent is injectedinto the vessel) using the ideal gas equation of state: P_(CO2) V=n R T,where Pro_(CO2)=partial pressure of CO₂ (psia), V=volume of the gasphase hi the vessel, n=total moles of CO₂ gas, T=measured experimental,temperature in degrees Kelvin, and R is the Ideal Gas Constant, which,has units of energy per mol per degree Kelvin (approximately 8.314 J/molK).

The water vapor pressure in the vessel is determined assuming thesolvent is an ideal mixture of water and absorbent using the known vaporpressure of water at the temperature of a given experiment (taken fromthe NISI steam tables, see http://webbook.nnist.gov/chemistry/fluid):the partial pressure of water in the vessel is calculated as the vaporpressure of water multiplied by the mol fraction of water in the solvent(which is known from the mass of water and absorbent used in preparingthe solvent mixture). Because the ionic materials used as absorbentshave negligibly low vapor pressure, the gas in the vessel is composedonly of water and CO₂. The partial pressure of CO₂ is calculated as thedifference between the measured total pressure and the water vaporpressure. The difference between the gas-phase quantity of CO₂ beforeand after solvent injection is used to calculate the absorbed quantityof CO₂ for various temperatures in a single experiment. The absorbedquantity of CO₂ is then divided by the quantity of ionic material in thesolvent to calculate the loading as moles of CO₂ absorbed per mole ofionic absorbent.

EXAMPLE 1

The effect of water addition to the ionic absorbents was studied usingtetraethylammonium N-isopropyl-N-(3-sulfopropyl)amine (TEA) as the ionicabsorbent. Neat TEA is a very viscous material. The addition of waterlowered the viscous nature of the TEA significantly. Varying amounts ofwater was added to the TEA and the time to reach the equilibrium CO₂uptake was determined. The results are summarized below in Table 1.

TABLE 1 Viscosity Water Diluent, Time to reach Viscosity (cSt) (cSt) Wt.% equilibrium CO₂ uptake at 20° C. at 80° C. 0 >>2 days Very viscous —25 ~24 hours Viscous — 50 <1 hour 10.5 1.7As the data show, a neat ionic absorbent (i.e., TEA) provided asignificantly longer time to approach equilibrium as compared to (1) theaqueous ionic absorbent solution containing TEA with 25 wt. % water and(2) the aqueous ionic absorbent solution containing TEA with 50 wt. %water, i.e., 2 days versus 24 hours and 1 hour, respectively. The lowerviscosity of the aqueous ionic absorbent, solution case facilitatesrapid mass transfer of CO₂ between the gas and liquid phases, andenables CO₂ absorption to occur on a faster timescale as compared to theneat ionic absorbent which is mass-transfer limited due to the highviscosity of the absorbent. These results demonstrate that sufficientdiluent content is necessary to achieve CO₂ removal from gas streams onan industrially-relevant timescale.

EXAMPLE 2

Two 50 wt. % aqueous ionic absorbent solutions of ionic absorbent wereprepared. The ionic absorbents were tetraediylammoniumN-isopropyl-N-(3-sulfopropyl)amine (TEA) and tetramethylammoniumN-isopropyl-N-(3-sulfopropyl)amine (TMA), respectively. The aqueousionic absorbent solution was contacted in a sealed vessel with a lowpartial pressure CO₂ gas simulating a model flue gas stream. The vesselwas maintained at a constant temperature and is shaken to allow goodcontact between CO₂ and the absorbent ionic absorbent solution until thesystem reached equilibrium. When the pressure reached the steady state,then a pressure was recorded and then the vessel temperature was changedto the next set point. Based on the equilibrium CO₂ pressure, theloading per mole of ionic absorbent was calculated. The CO₂ loading ofthe two ionic absorbents were plotted as a function of CO₂ partialpressure in FIG. 6. The data in this figure demonstrate that thesesolutions have high molar absorption capacities ranging from 0.5 toabout 0.85 CO₂ absorption loading per mole of ionic absorbent. Theseabsorption capacities are significantly higher than the conventionalamine-based solvents such as monoethanolamine (MEA). The CO₂ loading for30 wt. % MEA was also measured and the data are presented in FIG. 7which shows 0.4 to 0.5 mol CO₂ absorbed per mole of MEA (comparativeexample).

EXAMPLE 3

From the data in FIG. 6 the expected CO₂ removal capacity of the ionicabsorbent from flue gas can be estimated. The plot shows that an aqueousionic absorbent solution of TMA with 50 wt. % water can absorb up to0.74 mol CO₂ per mol ionic absorbent at 40° C. with a CO₂ partialpressure of 2.8 psia. This condition is reflective of the “rich”solution loading in the absorber. The solution absorption capacity forCO₂ at 95° C. and a CO₂ partial pressure of 5.0 psia decreases to 0.5mol CO₂ per mol aqueous ionic solution. This second condition mayreflect the “lean” solution loading achieved in the regeneration column.Therefore, the absorber-regenerator system described herein could remove0.24 moles of CO₂ per mol of TMA ionic absorbent circulated through theabsorber-regenerator system per pass.

Since the ionic absorbent for use in the aqueous ionic absorbentsolution can be operated in a wide range of temperatures, the CO₂capture process conditions can be chosen to increase the loadingdifference between the “rich” and “lean” CO₂ capacities. As one skilledin the art will readily appreciate, the chemical absorption capacity ofsolvent decreases with (1) decreasing partial pressure of the absorbedspecies in the gas phase and (2) increasing temperature. Therefore, byoperating the stripper at temperatures in excess of 95° C., the CO₂removal capacity of the system can be further increased. The removalcapacity of the system can also be increased by operating the stripperwith lower CO₂ partial pressure.

Therefore, our results show that that TMA would have the capacity toremove at least 0.24 moles of CO₂ per mol of ionic absorbent circulatedthrough the absorber-regenerator system per pass.

EXAMPLE 4

From the data in FIG. 6, the expected CO₂ removal capacity of TEA fromflue gas can be estimated. The plot shows that an aqueous ionicabsorbent solution of TEA with 50 wt. % water absorb up to 0.76 mol CO₂per mol ionic absorbent at 40° C. with a CO₂ partial pressure of 1.2psia. This condition is reflective of the “rich” solution loading in theabsorber. In this TEA-based solution, the absorption capacity for CO₂ at95° C. and a CO₂ partial pressure of 6.3 psia decreases to 0.57 mol CO₂per mol aqueous ionic solution. This second condition may reflect the“lean” solution loading achieved in the regeneration column. Therefore,the absorber-regenerator system described herein could remove 0.19 molesof CO₂ per mol of aqueous ionic absorbent solution circulated throughthe absorber-regenerator system. As in the previous example, the CO₂removal capacity of the system will be further increased by operatingthe stripper at higher temperatures and/or operating the stripper atlower CO₂ partial pressure.

Therefore, our results show that TEA would have the capacity to removeat least 0.19 moles of CO₂ per mol of ionic absorbent circulated throughthe absorber-regenerator system per pass.

Comparative Example A

FIG. 7 shows measured CO₂ absorption data for an aqueous solutioncontaining 30 wt % MEA with 70 wt. % water. At 20° C., the absorptioncapacity for CO₂ at a CO₂ partial pressure of 1.5 psia was approximately0.5 mol CO₂ per mol MEA, which reflects the loading in the absorptioncolumn. By heating the solution at 95° C. and a CO₂ partial pressure of3.0 psia, the measured loading decreases to 0.4 mol CO₂ per mol of MEA.This second condition reflects the “lean” solution loading achieved inthe regeneration column. Therefore, the absorber-regenerator systemusing an aqueous solution of MEA under these conditions could remove0.10 moles of CO₂ per mol of MEA. When comparing Comparative Example Awith Examples 2 and 3, the capacities of aqueous ionic absorbentsolution containing TEA and TMA, respectively, for CO₂ are higher thanthat of aqueous MEA, measured as moles of CO₂ captured per mol ofabsorbent.

It will be understood that various modifications may be made to theembodiments disclosed herein. Therefore the above description should notbe construed as limiting, but merely as exemplifications of preferredembodiments. For example, the functions described above and implementedas the best mode for operating the present invention are forillustration purposes only. Other arrangements and methods may beimplemented by those skilled in the art without departing from the scopeand spirit of this invention. Moreover, those skilled in the art willenvision other modifications within the scope and spirit of the claimsappended hereto.

1. A process for removal of CO₂ from a flue gas stream, the process comprising (a) contacting a flue gas stream containing water vapor and CO₂ with an ionic absorbent under absorption conditions to absorb at least a portion of the CO₂ from the flue gas stream and form a CO₂-absorbent complex; wherein the ionic absorbent comprises a cation and an anion comprising an amine moiety; and (b) recovering a gaseous product having a reduced CO₂ content.
 2. The process of claim 1, wherein the flue gas is a stack gas.
 3. The process of claim 1, wherein the flue gas comprises fully saturated water.
 4. The process of claim 1, wherein the cation comprises one or more cations selected from the group consisting of an ammonium cation, a phosphonium cation, a Group 1 metal cation and a Group 2 metal cation.
 5. The process of claim 1, wherein the cation comprises one or more of a secondary, tertiary or quaternary ammonium cation, or a secondary, tertiary or quaternary phosphonium cation.
 6. The process of claim 1, wherein the anion comprising an amine moiety is represented by the general formula: R¹—N(R¹)-(L)A⁻ wherein R¹ is the same or different and includes hydrogen, a straight or branched C₁ to C₃₀ substituted or unsubstituted alkyl group, a C₁ to C₂₀ ester-containing group, a substituted or unsubstituted C₃ to C₃₀ cycloalkyl group, a substituted or unsubstituted C₃ to C₃₀ cycloalkylalkyl group, a substituted or unsubstituted C₃ to C₃₀ cycloalkenyl group, a substituted or unsubstituted C₅ to C₃₀ aryl group, a substituted or unsubstituted C₅ to C₃₀ arylalkyl group, a substituted or unsubstituted C₅ to C₃₀ heteroaryl group, a substituted, or unsubstituted C₃ to C₃₀ heterocyclic ring, a substituted or unsubstituted C₄ to C₃₀ heterocyclolalkyl group, a substituted or unsubstituted C₆ to C₃₀ heteroarylalkyl group, or R and R¹ together with the nitrogen atom to which they are bonded are joined together to form a heterocyclic group; L is a linking group; and A⁻ is an anionic moiety.
 7. The process of claim 6, wherein R¹ is the same or different and is a straight or branched C₁ to C₆ substituted or unsubstituted alkyl group, L is a divalent straight or branched C₁ to C₆ substituted or unsubstituted alkyl group and A⁻ is SO₃ ⁻.
 8. The process of claim 1, wherein the ionic absorbent is selected from the group consisting of tetrabutylammonium N-propyl-N-(3-sulfopropyl)amine, tetrabutylphosphonium N-isopropyl-N-(3-sulfopropyl)amine, tetraethylammonium N-isopropyl-(3-sulfopropyl)amine and mixtures thereof.
 9. The process of claim 1, wherein the molecular weight of the ionic absorbent is from about 75 to about 700 atomic mass unit (AMU).
 10. The process of claim 1, further comprising the step of adding a diluent to the ionic absorbent to form an aqueous solution.
 11. The process of claim 1 wherein at least a portion of the CO₂-absorbent complex is subjected to desorption conditions to form a CO₂ effluent and a stream comprising recovered absorbent.
 12. The process of claim 1, wherein the flue gas stream further contains oxygen compounds, sulfur compounds and nitrogen compounds and the process further removes one or more of SO₂, COS, NO_(x), COS, and SO_(x).
 13. The process of claim 1, wherein the ionic absorbent absorbs higher than about 0.5 mol CO₂ per mol ionic absorbent at 40° C. with a CO₂ partial pressure in the range of about 1 to about 20 psia. 14.-23. (canceled)
 24. The process of claim 10, wherein the diluent is selected from the group consisting of water, a monohydric alcohol, a polyol, and mixtures thereof.
 25. The process of claim 10, wherein the aqueous solution comprises a total amount of water of from about 15 to about 80 wt % based on the total weight of the aqueous solution.
 26. The process of claim 10, wherein the aqueous solution comprises a total amount of water of from about 25 to about 80 wt. %, based on the total weight of the aqueous solution.
 27. The process of claim 10, wherein the aqueous solution comprises a total amount of water of from about 50 to about 80 wt. %, based on the total weight of the aqueous solution.
 28. The process of claim 10, wherein the aqueous solution has a viscosity of from about 0.5 to about 40 cSt.
 29. The process of claim 11, wherein the desorption conditions include a temperature of about 90° C. to about 200° C. 